In general, with semiconductor elements that are used for optical communications such as semiconductor laser elements and RF elements, the issue of how to efficiently dissipate heat generated from the elements is very important for preventing operation failures. In recent years, progress in the art of semiconductor elements has been accompanied by higher power, higher speed and higher integration of elements, placing stricter demands on the ability to dissipate heat. For this reason, high thermal conductivity is generally required in heat dissipating components such as heat sinks, so copper (Cu) which has a high thermal conductivity of 390 W/mK is used.
On the other hand, individual semiconductor elements have become larger in size with higher power, and the problem of mismatches between the thermal expansion of semiconductor elements and the heat sinks used for heat dissipation has become more apparent. In order to solve these problems, the development of a heat sink material having both the property of high thermal conductivity and a coefficient of thermal expansion matching that of semiconductor elements has been sought. As such materials, composites of metals and ceramics, such as composites of aluminum (Al) and silicon carbide (SiC), have been proposed (Patent Document 1).
However, no matter how the conditions are optimized in an Al—SiC composite, the thermal conductivity is 300 W/mK or less, so the development of a heat sink material having a thermal conductivity that is even higher than the thermal conductivity of copper has been sought. As such a material, a metal-diamond composite combining the high thermal conductivity of diamond and the high coefficient of thermal expansion of metals, having a high thermal conductivity and a coefficient of thermal expansion close to that of semiconductor element materials has been proposed (Patent Document 2).
Additionally, Patent Document 3 describes forming a β-type SiC layer on the surface of diamond particles to suppress the generation of metal carbides of low thermal conductivity formed during compositing and to improve the wettability with molten metals, thereby improving the thermal conductivity of metal-diamond composites.
Furthermore, since diamond is a very hard material, the metal-diamond composites obtained by compositing with metals are similarly very hard, and therefore difficult to work. For this reason, metal-diamond composites are almost unworkable with normal diamond tools, so in order to use metal-diamond composites as heat sinks which are compact and exist in various shapes, there is the issue of how to shape them at low cost. In response to this issue, laser machining and waterjet machining have been considered, and since metal-ceramic composites can pass electricity, methods of machining by electrical discharge have also been considered.
With heat dissipating components for use with semiconductor elements, a metal layer must be added to the surface of the heat dissipating component by coating or the like in order to enable them to be attached to the elements. In the case of normal semiconductor elements, bonding by solder is most common, with a bonding temperature of 300° C. or less, so a metal layer is provided on the surface by plating a Ni—P alloy or the like. However, regarding the manner of use of materials for heat sinks, heat sinks are usually arranged in contact with the semiconductor element by bonding with a brazing material in order to enable efficient dissipation of the heat generated by the semiconductor element. For this reason, multilayered plating having metal plating added to the bonding surface is used. Furthermore, with this manner of use, higher bonding temperatures and increases in the temperature load at the time of actual use can cause amorphous metals to crystallize in conventional alloy plating such as Ni—P alloys, and the change in volume can result in formation of microcracks, with the cracks being extended with subsequent temperature loads.
Furthermore, when a heat sink is bonded to a semiconductor element with brazing materials or the like, the planar precision of the bonding boundary is important for heat dissipation. In the case of conventional metal-diamond composites, diamond particles are exposed on the contact surface, making the contact surface rough, and consequently increasing the thermal resistance of the contact boundary which is undesirable. For this reason, there is also the issue of how to reduce the roughness of the surface as a property sought in heat sink materials.    Patent Document 1: JP H9-157773 A    Patent Document 2: JP 2000-303126 A    Patent Document 3: JP 2007-518875 A